Method for making flotable, particles suspended in a liquid by means of gas bubbles

ABSTRACT

A method for removing suspended particles from a liquid by means of gas-bubble flotation, in which, before the gas-bubble flotation step proper, the liquid is more or less saturated with the gas in one or more preceding gas introduction steps, so as to obtain a longer average life-time of the smaller gas bubbles in the final flotation step.

The invention relates to a method for making particles, which aresuspended in a liquid and are to be removed therefrom, flotatable bymeans of gas bubbles, which particles are made lighter than the liquidby adhesion of gas bubbles. This is, in particular, favourable forsuspended components having a specific weight which only slightlydiffers from that of the liquid, and/or when the separation thereof isconsiderably retarded by friction forces. Also in the case of particleswhich are heavier than the liquid, the separation sense can often bereversed in that manner.

To that end a liquid, and generally the cleaned carrier liquid, issaturated under pressure with a gas, in particular air, and isdepressurized just before being introduced into the liquid to betreated, a large number of gas bubbles then being generated, at least apart of which will attach themselves to particles suspended in theliquid. These particles then become considerably lighter than thecarrier liquid, and can, therefore, be separated from the liquid byflotation.

However the gas bubble adhesion will take place more difficultly as thegas bubbles are bigger in relation to the particles to be madeflotatable. Already when being generated these bubbles substantiallyvary in size, which variation is still increased in that these bubbleswill coalesce to form larger bubbles, and, thus, a limit is set to theflotation effect of the bubbles.

It is an object of the invention to improve the flotation effectobtainable with gas bubbles, and to provide a method to that end inwhich into a first liquid to be treated a second liquid is introducedwhich, at the pressure and temperature of the first liquid, issupersaturated with a gas, which method is characterized in that theintroduction of the second liquid is done in at least two successivesteps, and this in such a manner that the concentration of the dissolvedgas in the first liquid is increased first before, by a subsequentintroduction of the second liquid, a generation of the most effectivesmall gas bubbles is effected.

For it has appeared that the liquid to be treated is, generally,subsaturated or at best saturated with gas, in particlar air, so that,when introducing gas bubbles, at first the concentration of dissolvedgas in the liquid is increased until the gas pressure in the liquid moreor less corresponds to that in the gas bubbles, and only then asufficient gas bubble generation can occur. It has appeared that, whenintroducing the gas at once, smaller gas bubbles will be insufficientlygenerated, which is a consequence of this pressure difference. If, now,the gas is introduced in consecutive steps, a saturation of the liquidcan be obtained to such an extent that, in a subsequent step, asufficient number of gas bubbles suitable for flotation purposes will beobtained.

In particular this step-wise introduction of gas is obtained in a liquidwhich is continuously supplied to a separation device by means of a ductor pipe line by introducing the gas in this duct or pipe line indifferent consecutive points which are mutually separated in the liquidflow sense.

If, moreover, additional substances for promoting separation are to beadded to the liquid, this can be done with a simultaneous introductionof gas, which may lead to a better effect of these substances in theliquid to be treated.

The invention relates, furthermore, to a device which is designed forexecuting this method.

The invention will be described below by reference to a drawing, showingin:

FIG. 1 a diagrammatical plan view of an embodiment of a device accordingto the invention; and

FIG. 2 a transverse section on line II--II of FIG. 1.

Before describing the method and the device according to the invention,the backgrounds thereof will be considered first.

The invention is based on the insight that big gas bubbles behavedifferently from smaller ones. The internal pressure of a gas bubble inrespect of the liquid is, inter alia, dependent on the surface tensionof the liquid boundary layer surrounding the gas bubble. Furthermorethis pressure is of interest for the migration of the gas from thebubble into the liquid, for which, of course, the concentration of thegas dissolved in the liquid is important too. Generally speaking thispressure difference is proportional to 1/r, if r is the radius of thegas bubble under consideration. The surface area of the bubble isproportional to r² and its volume to r³. The diffusion of gas towardsthe surrounding liquid is proportional to the pressure difference andthe surface area, so that the loss of gas by diffusion related to thevolume of the bubble will, in first approximation, be proportional to(1/r)·r² /r³ =1/r². This means that small bubbles will dissolverelatively faster than bigger ones. Moreover the collision probabilitybetween two gas bubbles and the coalescence thereof into a bigger one isat least proportional to r₂, which probability is, for bigger bubbles,substantially larger than for smaller ones.

A consequence thereof is that, if initially a more or less uniformdistribution of the bubble sizes is present, the smaller ones willquickly disappear by dissolving, and the number of bigger ones willincrease by coalescence. Moreover, as the concentration of dissolved gasin the liquid increases, the bigger bubbles will take in gas again fromthe liquid, in particular since a certain concentration gradient ispresent around such bubbles.

On the other hand small bubbles will easier adhere to suspendedparticles than bigger ones. Starting with a given size distribution ofthe bubbles and suspended particles, one might make an estimate of theadhesion probability at a given residence time. This is, however,traversed by the short life of the smaller gas bubbles, which,eventually, restricts the period suitable for attachment veryconsiderably. Also the bubbles already attached to particles maydisappear again by dissolving, and are, then, lost for the flotationeffect.

The invention is, now, based on the insight that, by dividing the airbubble introduction in partial steps, the number of active air bubblescan be increased, and this because:

(1) at each renewed introduction small bubbles will be generated again,which are, partly, available for attaching themselves to suspendedparticles; and

(2) after each introduction of gas bubbles the gas content of the liquidincreases, so that successively smaller bubbles can survive in theliquid witout being quickly dissolved.

It is, of course, possible to treat a quantity of liquid present in avessel with gas bubbles at intervals, so that, then, the gasificationsteps mentioned above are distributed in time. However, in a continuoustreatment, e.g. at the entry side of a separation device, this will beobjectionable, so that, then, a stepwise introduction of gas willpreferably be obtained by providing additional injection points in thesupply duct or pipe line towards the separator, which points aresituated at mutual distances in the flow sense.

In FIGS. 1 and 2 an embodiment of such a device is shown in adiagrammatical manner. The device shown comprises a basis 1 in which aplate separator 2 is arranged at an angle in respect of the horizontalplane. By means of a partition 3 this basin is divided into a supplychamber 4 and a discharge chamber 5, the part 6 of said supply chamber 4above the inclined upper surface of the plate separator 2 having atriangular cross-section. At the other side of the basin 1 an overflowweir 7 is arranged, over which a floating layer 8 can flow off towards adischarge trough 9, the latter communicating with a discharge duct 10,and, if necessary, skimmers or the like may be used for moving the layer8 towards the weir 7. The discharge chamber 5, on the other hand, isprovided with an overflow weir 11 for determining the liquid level inthe basin 1, followed by a discharge trough 12 and a discharge duct 13for the treated liquid.

Near the deepest point of the space 6 a supply tube 14 is situated whichis provided with continuous or interrupted outflow slots 15. Outside thebasin 1 the tube 14 communicates with a supply duct 16, and a supplynozzle 17 extends into the tube 14, which nozzle is connected, by meansof a relief valve 18, to a duct 19 which is connected to a pump notshown, by means of which a pressurized gas, in particular air, issupplied to the liquid. In this manner a large number of gas bubbles isgenerated in the liquid supplied through the duct 16 by the abruptpressure relief of the liquid saturated with gas, which bubbles canattach themselves to the particles suspended in the liquid, and thenthese particles will flotate towards the floating layer 8. Subsequentlythe liquid flows through the separator 2, in which the remaining lighterparticles and gas bubbles will be separated, and possibly presentheavier particles will sedimentate there and will be collected in afunnel 19. The clean liquid flows off over the weir 11 towards thedischarge duct 13.

It has now appeared that the separation effect can be very considerablyimproved by providing, in the duct 16, additional injection nozzles 20and associated relief valves 21 which are connected to the pressure line19 as well. The relief valve 18 can, accordingly, be adjusted moresparingly so as to reduce the quantity of pressurized liquid introducedby the nozzle 17 in accordance with the quantity introduced by thenozzles 20. The distance between the additional nozzles 20 is, then,chosen in such a manner that, taking into account the flow velocity ofthe liquid in the duct 16, there will be sufficient time for dissolvingthe gas in the liquid, thereby improving the gas saturation degree.

Experiments have shown that the flotation efficiency of the gas bubbles,i.e. the ratio between the amounts of gas attached to suspendedparticles and the total amount of supplied gas, can be substantiallyimproved in this manner, so that even a substantially smaller amount ofpressurized liquid can be used. This leads to accordingly substantialenergy savings, and, moreover, the compression pump may be made smallerwhich will lead to corresponding cost reductions too.

In the duct 16 often a construction 22 is present, and a supply 23 opensin said duct 16 at the upstream side thereof for introducing a flotationpromoting agent, e.g. a polyelectrolyte. It has appeared that, if anadditional gas injection nozzle 16' with relief valve 17' is arranged insaid supply 23, a considerably better effect will be obtained. If theagent thus supplied induces a lowering of the surface tension, theoverpressure in the gas bubbles will be lowered accordingly, so that thedissolution tendency of the gas bubbles is reduced. This can have anadditional beneficial influence on the separation effect of the gasbubbles.

A further advantage is that the relief valve 13, which, in the existingdevices, is designed for passing a relatively large amount of liquidsaturated with gas, can be made substantially smaller when using themethod of the invention. This too will lead to additional savings.

It will be clear that the invention is not restricted to the embodimentdescribed above. For instance the use of a plate separator 2 is notrequired if, with gas bubbles alone, a sufficient flotation of thecomponents to be removed is obtained. Furthermore the supply ofadditional substances can take place in other points, for instance inthe basin 1 itself.

Sometimes it will be favourable to use an open trough for at least apart of the duct 16. The advantage thereof is that, then, the effect ofthe pressurized liquid introduced by means of the nozzles 20 is directlyvisible, which will simplify the adjustment of the relief valves 21.

Within the scope of the invention many other modifications are possible.

I claim:
 1. A flotation method for separating particles suspended in afirst carrier liquid to be treated by adhesion of gas bubbles to saidparticles in which said first liquid to be treated is conveyed along afirst flow path towards an outlet in a flotation chamber and a secondliquid which is saturated at an elevated pressure with a gas is conveyedalong a second flow path, and is introduced after being depressurized tothe pressure of the first liquid into said first liquid at a first inletnear said outlet, the abrupt pressure reduction causing the formation ofsmall gas bubbles in said first liquid for flotating suspended particlesby adherence to gas bubbles; the method comprising the steps ofintroducing the second liquid into the first flow path afterdepressurization to the pressure of the first liquid at a plurality ofadditional inlets spaced along the length of the first flow pathupstream of said first inlet to produce gas bubbles in said first liquidto ensure that the first liquid is sufficiently saturated with gasbefore the second liquid is introduced into the first liquid at saidfirst inlet, thereby prolonging the life of the small bubbles toincrease flotation of the particles.
 2. The method of claim 1 in which aflotation promoting substance is introduced into said first flow pathnear one of said additional inlets for dispersion into said first liquidthrough means of the gas bubbles produced at said one of said one ofsaid additional inlets.